Method of performing uplink synchronization in wireless communication system

ABSTRACT

A method of performing uplink synchronization in a wireless communication system includes transmitting a random access preamble which is randomly selected from a set of random access preambles, receiving a random access response, the random access response comprising a random access preamble identifier corresponding to the random access preamble and a time alignment value for uplink synchronization, starting a time alignment timer after applying the time alignment value, starting a contention resolution timer after receiving the random access response, wherein contention resolution is not successful when the contention resolution timer is expired, and stopping the time alignment timer when the contention resolution timer is expired.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/141,772, filed Jun. 18, 2008, now U.S. Pat. No. 8,107,456, whichclaims the benefit of U.S. Provisional application Ser. No. 60/944,785,filed on Jun. 18, 2007, and which also claims benefit of earlier filingdate and right of priority to Korean Patent Application No.10-2008-0023809, filed on Mar. 14, 2008, which are all incorporated byreference herein in their entirety.

BACKGROUND

1. Technical Field

The present invention relates to wireless communications, and moreparticularly, to a method of performing uplink synchronization in awireless communication system.

2. Related Art

Third generation partnership project (3GPP) mobile communication systemsbased on a wideband code division multiple access (WCDMA) radio accesstechnology are widely spread all over the world. High-speed downlinkpacket access (HSDPA) that can be defined as a first evolutionary stageof WCDMA provides 3GPP with a radio access technique that is highlycompetitive in the mid-term future. However, since requirements andexpectations of users and service providers are continuously increasedand developments of competing radio access techniques are continuouslyin progress, new technical evolutions in 3GPP are required to securecompetitiveness in the future. Reduction of cost per bit, increase ofservice availability, flexible use of frequency bands, simple structureand open interface, proper power consumption of a user equipment (UE),and the like are defined as requirements.

In general, there are one or more cells within the coverage of a basestation (BS). One cell may include a plurality of UEs. A UE is generallysubjected to a random access procedure to access a network. The randomaccess procedure is performed by the UE for the following purposes: (1)initial access; (2) handover; (3) scheduling request; and (4) timingsynchronization. However, this is for exemplary purposes only, and thusthe content or number of purposes for performing the random accessprocedure may vary depending on systems.

The random access procedure can be classified into a contention basedrandom access procedure and a non-contention based random accessprocedure. Major difference between the two random access procedureslies in whether a random access preamble is dedicatedly assigned to oneUE. In the non-contention based access procedure, since a UE uses onlythe random access preamble dedicatedly assigned to the UE, contention(or collision) with another UE does not occur. The contention occurswhen two or more UEs attempt the random access procedure by using thesame random access preamble through the same resource. In the contentionbased random access procedure, there is a possibility of contentionsince a random access preamble used by the UEs is randomly selected.

In an orthogonal frequency division multiplexing (OFDM)-based wirelesscommunication system, timing synchronization between a UE and a BS isimportant so as to minimize interference between users. The randomaccess procedure is performed for uplink synchronization. While therandom access procedure is performed, the UE is time-synchronizedaccording to a time alignment value transmitted from the BS. When uplinksynchronization is achieved, the UE runs a time alignment timer. If thetime alignment timer is running, it is regarded that the UE and the BSare uplink-synchronized with each other. If the time alignment timer isexpired or is not running, it is regarded that the UE and the BS are notsynchronized with each other. In this case, uplink transmission cannotbe achieved except for transmission of the random access preamble.

A random access failure may occur in the contention based random accessprocedure since there is always a possibility of contention. Therefore,a method is needed for performing uplink synchronization caused by therandom access failure.

SUMMARY

The present invention provides a method of performing uplinksynchronization while a contention based random access procedure isperformed in a wireless communication system.

The present invention also provides a method of avoiding interference toother user equipments due to incorrect uplink synchronization in awireless communication system.

In an aspect, a method of performing uplink synchronization in awireless communication system is provided. The method includestransmitting a random access preamble which is randomly selected from aset of random access preambles, receiving a random access response, therandom access response comprising a random access preamble identifiercorresponding to the random access preamble and a time alignment valuefor uplink synchronization, starting a time alignment timer afterapplying the time alignment value, starting a contention resolutiontimer after receiving the random access response, wherein contentionresolution is not successful when the contention resolution timer isexpired, and stopping the time alignment timer when the contentionresolution timer is expired.

The method can further include transmitting a scheduled message, thescheduled message comprising a unique identifier and stopping thecontention resolution timer when receiving a contention resolutionmessage, the contention resolution message comprising an identifiercorresponding to the unique identifier. The contention resolution timermay be started when transmitting the scheduled message.

In another aspect, a method of performing uplink synchronization in awireless communication system is provided. The method includestransmitting a random access preamble which is randomly selected from aset of random access preambles, receiving a random access response, therandom access response comprising a random access preamble identifiercorresponding to the random access preamble, starting a time alignmenttimer after receiving a random access response and stopping the timealignment timer when contention resolution is not successful.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a wireless communication system.

FIG. 2 is a diagram showing functional split between an evolveduniversal terrestrial radio access network (E-UTRAN) and an evolvedpacket core (EPC).

FIG. 3 is a block diagram showing constitutional elements of a userequipment (UE).

FIG. 4 is a diagram showing a radio protocol architecture for a userplane.

FIG. 5 is a diagram showing a radio protocol architecture for a controlplane.

FIG. 6 is a flow diagram showing a random access procedure.

FIG. 7 is a flow diagram showing a method of performing uplinksynchronization according to an embodiment of the present invention.

FIG. 8 is a flow diagram showing a method of performing uplinksynchronization according to another embodiment of the presentinvention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a structure of a wireless communication system. Thewireless communication system may have a network structure of anevolved-universal mobile telecommunications system (E-UMTS). The E-UMTSmay be referred to as a long-term evolution (LTE) system. The wirelesscommunication system can be widely deployed to provide a variety ofcommunication services, such as voices, packet data, etc.

Referring to FIG. 1, an evolved-UMTS terrestrial radio access network(E-UTRAN) includes at least one base station (BS) 20 which provides acontrol plane and a user plane.

A user equipment (UE) 10 may be fixed or mobile, and may be referred toas another terminology, such as a mobile station (MS), a user terminal(UT), a subscriber station (SS), a wireless device, etc. The BS 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc. There are one ormore cells within the coverage of the BS 20. Interfaces for transmittinguser traffic or control traffic may be used between the BSs 20.Hereinafter, downlink is defined as a communication link from the BS 20to the UE 10, and uplink is defined as a communication link from the UE10 to the BS 20.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC), more specifically, to a mobility management entity (MME)/servinggateway (S-GW) 30. The S1 interface supports a many-to-may relationbetween the BS 20 and the MME/S-GW 30.

FIG. 2 is a diagram showing functional split between the E-UTRAN and theEPC.

Referring to FIG. 2, slashed boxes indicate radio protocol layers andwhite boxes indicate functional entities of the control plane.

The BS performs the following functions: (1) functions for radioresource management (RRM) such as radio bearer control, radio admissioncontrol, connection mobility control, and dynamic allocation ofresources to the UE; (2) Internet protocol (IP) header compression andencryption of user data streams; (3) routing of user plane data to theS-GW; (4) scheduling and transmission of paging messages; (5) schedulingand transmission of broadcast information; and (6) measurement andmeasurement reporting configuration for mobility and scheduling.

The MME performs the following functions: (1) distribution of pagingmessages to the BSs; (2) security control; (3) idle state mobilitycontrol; (4) system architecture evolution (SAE) bearer control; and (5)ciphering and integrity protection of non-access stratum (NAS)signaling.

The S-GW performs the following functions: (1) termination of a userplane packet for paging; and (2) user plane switching for the support ofUE mobility.

FIG. 3 is a block diagram showing constitutional elements of the UE. AUE 50 includes a processor 51, a memory 52, a radio frequency (RF) unit53, a display unit 54, and a user interface unit 55. Layers of a radiointerface protocol are implemented in the processor 51. The processor 51provides the control plane and the user plane. The function of eachlayer can be implemented in the processor 51. The memory 52 is coupledto the processor 51 and stores an operating system, applications, andgeneral files. The display unit 54 displays a variety of information ofthe UE 50 and may use a well-known element such as a liquid crystaldisplay (LCD), an organic light emitting diode (OLED), etc. The userinterface unit 55 can be configured with a combination of well-knownuser interfaces such as a keypad, a touch screen, etc. The RF unit 53 iscoupled to the processor 51 and transmits and/or receives radio signals.

Layers of a radio interface protocol between the UE and the network canbe classified into L1 layer (a first layer), L2 layer (a second layer),and L3 layer (a third layer) based on the lower three layers of the opensystem interconnection (OSI) model that is well-known in a communicationsystem. A physical layer, or simply a PHY layer, belongs to the firstlayer and provides an information transfer service through a physicalchannel. A radio resource control (RRC) layer belongs to the third layerand serves to control radio resources between the UE and the network.The UE and the network exchange RRC messages via the RRC layer.

FIG. 4 is a diagram showing a radio protocol architecture for the userplane. FIG. 5 is a diagram showing a radio protocol architecture for thecontrol plane. They illustrate an architecture of a radio interfaceprotocol between the UE and the E-UTRAN. The user plane is a protocolstack for user data transmission. The control plane is a protocol stackfor control signal transmission.

Referring to FIGS. 4 and 5, a PHY layer belongs to the first layer andprovides an upper layer with an information transfer service through aphysical channel. The PHY layer is coupled with a medium access control(MAC) layer, i.e., an upper layer of the PHY layer, through a transportchannel. Data is transferred between the MAC layer and the PHY layerthrough the transport channel. Between different PHY layers (i.e., a PHYlayer of a transmitter and a PHY layer of a receiver), data aretransferred through the physical channel. The PHY layer can be modulatedby orthogonal frequency division multiplexing (OFDM). Time and/orfrequency can be utilized as radio resources.

The MAC layer belongs to the second layer and provides services to aradio link control (RLC) layer, i.e., an upper layer of the MAC layer,through a logical channel. The RLC layer in the second layer supportsreliable data transfer. There are three operating modes in the RLClayer, that is, a transparent mode (TM), an unacknowledged mode (UM),and an acknowledged mode (AM) according to a data transfer method. An AMRLC provides bidirectional data transmission services and supportsretransmission when the transfer of a RLC protocol data unit (PDU)fails.

A packet data convergence protocol (PDCP) layer belongs to the secondlayer and performs a header compression function. When transmitting anIP packet such as an IPv4 packet or an IPv6 packet, a header of the IPpacket may contain relatively large and unnecessary control information.The PDCP layer reduces a header size of the IP packet so as toefficiently transmit the IP packet.

A radio resource control (RRC) layer belongs to the third layer and isdefined only in the control plane. The RRC layer serves to control thelogical channel, the transport channel, and the physical channel inassociation with configuration, reconfiguration and release of radiobearers (RBs). A RB is a service provided by the second layer for datatransmission between the UE and the E-UTRAN. When an RRC connection isestablished between an RRC layer of the UE and an RRC layer of thenetwork, it is called that the UE is in an RRC connected mode. When theRRC connection is not established yet, it is called that the UE is in anRRC idle mode.

A non-access stratum (NAS) layer belongs to an upper layer of the RRClayer and serves to perform session management and mobility management.

A downlink transport channel is a channel through which data istransmitted from the network to the UE. Examples of the downlinktransport channel include a broadcast channel (BCH) for transmittingsystem information and a downlink-shared channel (DL-SCH) fortransmitting user traffic or control messages. User traffic of downlinkmulticast (or broadcast) services or control messages can be transmittedon the DL-SCH or a downlink multicast channel (MCH). An uplink transportchannel is a channel through which data is transmitted from the UE tothe network. Examples of the uplink transport channel include a randomaccess channel (RACE) for transmitting initial control messages and anuplink-shared channel (UL-SCH) for transmitting user traffic or controlmessages.

A downlink physical channel is mapped to the downlink transport channel.Examples of the downlink physical channel include a physical broadcastchannel (PBCH) for transmitting information of the BCH, a physicalmulticast channel (PMCH) for transmitting information of the MCH, aphysical downlink shared channel (PDSCH) for transmitting information ofthe PCH and the DL-SCH, and a physical downlink control channel (PDCCH)for transmitting control information such as downlink or uplinkscheduling grant, which are provided from the first layer and the secondlayer. The PDCCH is also referred to as a downlink L1/L2 controlchannel. An uplink physical channel is mapped to the uplink transportchannel. Examples of the uplink physical channel include a physicaluplink shared channel (PUSCH) for transmitting information of theUL-SCH, a physical random access channel (PRACH) for transmittinginformation of the EACH, and a physical uplink control channel (PUCCH)for transmitting control information such as hybrid automatic repeatrequest (HARQ) acknowledgement (ACK)/non-acknowledgement (NACK) signals,a scheduling request signal, and a channel quality indicator (CQI),which are provided from the first layer and the second layer.

Now, the random access procedure will be described. A UE performs therandom access procedure in the process of performing the followingoperations, such as, (1) initial access, (2) handover, (3) transmissionof downlink data to a non-synchronized UE, (4) transmission of uplinkdata by the non-synchronized UE, and (5) restoration of radio linkfailure.

FIG. 6 is a flow diagram showing the random access procedure.

Referring to FIG. 6, in step S110, a UE transmits a random accesspreamble to a BS through a selected random access resource by usingsystem information received from the BS. The system information includesinformation on a set of available random access preambles. The randomaccess preamble transmitted by the UE is selected from the set of randomaccess preambles.

In step S120, the ES transmits a random access response through aDL-SCH. The random access response includes a time alignment value foruplink synchronization of the UE, uplink radio resource allocationinformation, an index of the random access preamble received to identifythe UE performing the random access procedure, and a temporaryidentifier (e.g., temporary cell-radio network temporary identity(C-RNTI)) of the UE.

In step S130, the UE applies the time alignment value, and transmits ascheduled message including a unique identifier of the UE to the BS byusing the uplink radio resource allocation information. The uniqueidentifier of the UE may be the C-RNTI, a SAE temporary mobile stationidentifier (S-TMSI), or an upper-layer identifier. The unique identifieris also referred to as a contention resolution identifier since it isused for contention resolution.

In step S140, after receiving the scheduled message, the BS transmits tothe UE a contention resolution message including the unique identifierof the UE.

Due to the limited number of available random access preambles,contention occurs in the random access procedure. Since it is notpossible to assign unique random access preambles to all UEs located ina cell, the UEs select one random access preamble from the set of theavailable random access preambles and then transmit the selected randomaccess preamble. Accordingly, two or more UEs can select and transmitthe same random access preamble through the same random access resource.This is a case where contention occurs. Upon receiving the random accesspreamble, the BS transmits the random access response for the randomaccess preamble in a state where the BS does not know the occurrence ofcontention. However, since contention has occurred, two or more UEsreceive the same random access response and thus transmit scheduledmessages according to information included in the random accessresponse. That is, the two or more UEs transmit different scheduledmessages according to the uplink radio resource allocation informationincluded in the random access response. In this case, the BS may fail toreceive all of the scheduled messages, or may successfully receive onlya scheduled message of a specific UE according to the location ortransmit (Tx) power of the UEs. If the BS successfully receives thescheduled message, the BS transmits the contention resolution message byusing the unique identifier of the UE, wherein the unique identifier isincluded in the scheduled message. The UE can know that the contentionresolution is successful when the unique identifier of the UE isreceived. The contention resolution allows the UE to know whether thecontention is successful or not in the contention based random accessprocedure.

A contention resolution timer is used for the contention resolution. Thecontention resolution timer starts after the random access response isreceived. The contention resolution timer may start when the UEtransmits the scheduled message. When the contention resolution timer isexpired, it is determined that the contention resolution isunsuccessful, and thus a new random access procedure is performed. Whenthe UE receives the contention resolution message including the uniqueidentifier of the UE, the contention resolution timer stops, and the UEdetermines that the contention resolution is successful. If the UEalready has a unique cell identifier (e.g., C-RNTI) before the randomaccess procedure is performed, the UE transmits the scheduled messageincluding the cell identifier of the UE and then starts the contentionresolution timer. If the UE receives a PDCCH, which is addressed by thecell identifier of the UE, before the contention resolution timer isexpired, the UE determines that the contention is successful and thenfinishes the random access procedure without errors. If the UE does nothave the C-RNTI, the upper-layer identifier may be used as the uniqueidentifier. After transmitting the scheduled message including theupper-layer identifier, the UE starts the contention resolution timer.If the contention resolution message including the upper-layeridentifier of the UE is received on the DL-SCH before the contentionresolution timer is expired, the UE determines that the random accessprocedure is successful. The contention resolution message is receivedby using the PDCCH addressed by the temporary C-RNTI. Otherwise, if theaforementioned contention resolution is not received on the DL-SCH untilthe contention resolution timer is expired, the UE determines that thecontention is unsuccessful.

Now, time alignment for uplink synchronization will be described. In anOFDM-based system, timing synchronization between a UE and a BS isimportant so as to minimize interference between users.

The random access procedure is one of uplink time synchronizationmethods. That is, the BS measures the time alignment value through therandom access preamble transmitted by the UE, and provides the timealignment value to the UE through the random access response. Uponreceiving the random access response, the UE applies the time alignmentvalue and starts the time alignment timer. Time synchronization betweenthe UE and the BS is maintained while the time alignment timer isrunning. If the time alignment timer is expired or is not running, it isregarded that the time synchronization between the UE and the BS is notmaintained. If the time alignment timer is expired and is not running,the UE can transmit only the random access preamble, and any otheruplink transmission cannot be achieved.

When contention occurs in the process of performing the random accessprocedure, the UE may apply an incorrect time alignment value. If the UEis not time-synchronized with the BS before the UE transmits the randomaccess preamble, the random access preamble may be mistakenlytransmitted in uplink because of the time alignment timer currentlyrunning.

First, in a state whether uplink synchronization is not achieved betweenthe UE and the BS, the UE transmits to the BS the random access preamblewhich is randomly selected, and receives the random access response forthe random access preamble. Even if contention occurs, the UE canreceive the random access response. In this case, since the UE cannotknow the occurrence of contention, the UE applies to the UE itself thetime alignment value included in the received random access response,and starts the time alignment timer. Subsequently, the UE transmits tothe BS the scheduled message including the unique identifier of the UEand starts the contention resolution timer. If the UE does not receivethe contention resolution message addressed by the unique identifier ofthe UE until the contention resolution timer is expired, the UE retriesthe random access procedure. However, since the time alignment timer iscontinuously running, the UE may transmit uplink data when the BStransmits downlink data. This is because the UE determines that uplinksynchronization is achieved by the time alignment timer currentlyrunning, even in a case where uplink synchronization is not achieved.The incorrect uplink synchronization may result in interference to otherusers during transmission.

FIG. 7 is a flow diagram showing a method of performing uplinksynchronization according to an embodiment of the present invention. Atfirst, a UE is in a state where a time alignment timer is expired or isnot running. This is a case where a contention based random accessprocedure starts when the UE attempts initial network entry or when acell is searched for again due to a radio link failure.

Referring to FIG. 7, in step S210, the UE transmits to a BS a randomaccess preamble which is randomly selected. In step S220, in response tothe random access preamble, the BS transmits a random access response tothe UE. The random access response includes uplink radio resourceallocation information, a random access preamble identifier, a timealignment value, and a temporary C-RNTI. In step S230, the UE appliesthe time alignment value included in the random access response, andstarts the time alignment timer.

In step S240, through the uplink radio resource allocation informationincluded in the random access response, the UE transmits to the BS ascheduled message including a unique identifier of the UE. In step S250.the UE transmits the scheduled message and then starts a contentionresolution timer.

In step S260, if the UE does not receive a contention resolution messageincluding the unique identifier of the UE until the contentionresolution timer is expired, the UE stops the time alignment timer. Inthis case, the UE may determine that the contention is unsuccessful, andthus retry the random access procedure.

If the random access failure occurs, the UE stops the time alignmenttimer which has been previously running. This is because the previouslyreceived time alignment value can be for another UE. Therefore, sincethe time alignment timer is stopped, uplink transmission is preventedfrom using the incorrect time alignment value.

FIG. 8 is a flow diagram showing a method of performing uplinksynchronization according to another embodiment of the presentinvention.

Referring to FIG. 8, in step S310, a UE transmits to a BS a randomaccess preamble which is randomly selected. In step S320, in response tothe random access preamble, the BS transmits a random access response tothe UE. The random access response includes a time alignment value andtemporary C-RNTI. In step S330, the UE applies the time alignment valueand starts a time alignment timer. In step S340, the UE transmits to theBS a scheduled message including a unique identifier of the UE throughuplink radio resource allocation information included in the randomaccess response. In step S350, after transmitting the scheduled message,the UE starts a contention resolution timer.

In step S360, before the contention resolution timer is expired, the UEreceives a contention resolution message indicated by PDCCH addressed bythe temporary C-RNTI. The contention resolution message includes acontention resolution identifier. In step S370, the UE determineswhether the contention is successful through the contention resolutionidentifier. For example, the contention resolution identifier can becompared with the identifier included in the scheduled message. If thetwo identifiers are not identical, it is determined that the contentionis unsuccessful.

In step S380, if it is determined that the contention is unsuccessful,the UE stops the time alignment timer. In this case, the UE may retrythe random access procedure or may report to an upper layer that thecontention is unsuccessful.

If the contention is unsuccessful in the random access procedure, the UEstops the time alignment timer currently running. Accordingly, a problemcan be solved in which the time alignment timer continuously runs evenwhen uplink synchronization is not achieved.

Incorrect uplink synchronization can be avoided even if contentionresolution is unsuccessful. Therefore, interference to other userequipments can be mitigated and service delay due to incorrect uplinksynchronization can be prevented.

The steps of a method described in connection with the embodimentsdisclosed herein may be implemented by hardware, software or acombination thereof. The hardware may be implemented by an applicationspecific integrated circuit (ASIC) that is designed to perform the abovefunction, a digital signal processing (DSP), a programmable logic device(PLD), a field programmable gate array (FPGA), a processor, acontroller, a microprocessor, the other electronic unit, or acombination thereof. A module for performing the above function mayimplement the software. The software may be stored in a memory unit andexecuted by a processor. The memory unit or the processor may employ avariety of means that is well known to those skilled in the art.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims. Therefore, allchanges and modifications that fall within the metes and bounds of theclaims, or equivalence of such metes and bounds are intended to beembraced by the appended claims.

What is claimed:
 1. A method of performing uplink synchronization in awireless communication system, the method performed by a user equipment(UE) and comprising: transmitting a random access preamble from the UEto a network; receiving at the UE a random access response from thenetwork, the random access response comprising a temporary UE identifierand a time alignment value for uplink synchronization with the network;starting a first timer in the UE responsive to receiving of the timealignment value; starting a second timer in the UE after receiving therandom access response; determining whether contention resolution issuccessful by the UE, wherein the contention resolution is notsuccessful when the second timer expires; and stopping the first timerin the UE if the contention resolution is not successful.
 2. The methodof claim 1, wherein the first timer is a time alignment timer.
 3. Themethod of claim 1, wherein the second timer is a contention resolutiontimer.
 4. The method of claim 1, further comprising: transmitting by theUE a scheduled message, the scheduled message comprising a uniqueidentifier; and stopping the second timer in the UE when receiving acontention resolution message, the contention resolution messagecomprising a contention resolution identifier associated with the uniqueidentifier.
 5. The method of claim 4, wherein the second timer isstarted once the scheduled message is transmitted from the UE.
 6. Themethod of claim 4, wherein the random access response further comprisesuplink radio resource assignment, and the scheduled message istransmitted in the uplink radio resource assignment.
 7. The method ofclaim 4, wherein the random access response further comprises uplinkradio resource assignment, and the scheduled message is transmitted inthe uplink radio resource assignment.
 8. The method of claim 4, whereinthe UE determines that contention resolution is successful when thecontention resolution identifier matches with the unique identifier. 9.The method of claim 1, wherein the temporary UE identifier is atemporary Cell Radio Network Temporary Identifier (C-RNTI).
 10. A userequipment (UE) for performing uplink synchronization in a wirelesscommunication system, the UE comprising: a radio frequency (RF) unitconfigured for: transmitting a random access preamble to a network;receiving a random access response from the network, the random accessresponse comprising a temporary UE identifier and a time alignment valuefor uplink synchronization with the network; starting a first timer inthe UE responsive to receiving of the time alignment value; starting asecond timer in the UE after receiving the random access response;determining whether contention resolution is successful, wherein thecontention resolution is not successful when the second timer expires;and stopping the first timer in the UE if the contention resolution isnot successful.
 11. The user equipment of claim 10, wherein the firsttimer is a time alignment timer.
 12. The user equipment of claim 10,wherein the second timer is a contention resolution timer.
 13. The userequipment of claim 10, wherein the RF unit is further configured for:transmitting a scheduled message, the scheduled message comprising aunique identifier; and stopping the second timer in the UE whenreceiving a contention resolution message, the contention resolutionmessage comprising a contention resolution identifier associated withthe unique identifier.
 14. The user equipment of claim 13, wherein thesecond timer is started once the scheduled message is transmitted fromthe UE.
 15. The user equipment of claim 13, wherein the random accessresponse further comprises uplink radio resource assignment, and thescheduled message is transmitted in the uplink radio resourceassignment.
 16. The user equipment of claim 13, wherein the randomaccess response further comprises uplink radio resource assignment, andthe scheduled message is transmitted in the uplink radio resourceassignment.
 17. The user equipment of claim 13, wherein the RF unit isfurther configured to determine that contention resolution is successfulwhen the contention resolution identifier matches with the uniqueidentifier.
 18. The user equipment of claim 10, wherein the temporary UEidentifier is a temporary Cell Radio Network Temporary Identifier(C-RNTI).